How to disable KDE's brightness OSD?

I am using lightum to automatically control the screen's brightness depending on the surrounding light (on a Retina MacBook Pro).

The problem is, though, that KDE picks up the change and displays the OSD (same one that shows up when you change brightness with keyboard).

The brightness changes slightly every minute or so, making the OSD incredibly annoying. I would gladly get rid of it entirely if I could.

Some research

I looked in KDE's notifications settings, found nothing. I also grep'd ~/.kde/share/config, found nothing. Google didn't help either.

Apparently setting the brightness by doing

# echo 4 > /sys/devices/pci0000\:00/0000\:00\:02.0/backlight/acpi_video0/brightness

doesn't bring up the OSD. From the source code, lightum seems to be setting brightness via dbus (org.kde.Solid.PowerManagement.setBrightness).

You can do that by going into the Global Shortcuts control panel in System Settings and, under the "kded" section, remove the key binding for brightness up and brightness down.